Introduction:
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Define a mobile management strategy that fits the needs of your organization. Apply flexible mobile device and app management controls that let employees work with the devices and apps they choose while protecting your company information. [1]

Compass Security discovered a design weakness in Microsoft Intune's iOS Keychain management. This allows users to access company data even after the device has been unenrolled.

Technical Description
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If a user's device, which is enrolled with their company's MDM, is unenrolled, their Office access tokens are not removed from the iOS Keychain. Furthermore, the respective tokens are not invalidated on the server-side. Therefore, if the user reinstalls Office to their device after unenrollment, they may again obtain full access to the company's files.

Workaround / Fix:
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This issue can be fixed by invalidating the user's access token on the server- and client-side. In addition, the Keychain items could also be encrypted with a key stored in the app's data directory. Since this key is removed with the data directory on uninstallation of the app, this renders the Keychain entry useless.

Timeline:
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2017-08-22 Discovery by Stephan Sekula
2017-09-17 Initial vendor notification
2017-09-18 Initial vendor response
2017-10-04 Asking vendor for update
2017-10-04 Vendor replies that engineers are working on reproducing the issue
2017-11-01 Asking vendor for an update
2017-11-02 Vendor replies - They are waiting for a partner team to respond on the case.
2018-01-08 Asking vendor for update - No response
2018-02-12 Asking vendor for update - No response
2018-03-19 Public disclosure